Treatment of used chromic acid etching solutions by extraction with acetone



p 1970 R. R. DOUGHERTY 3,531,

TREATMENT OF USED CHRONIC ACID ETCHING SOLUTIONS BY EXTRACTION WITH ACETONE Filed Feb. 23. 1968 INVENTOR Ross/qr R. DOUGHERTY A T TOPNEYS United States Patent Oflice 3,531,262 Patented Sept. 29 1970 TREATMENT OF USED CHROMIC ACID ETCHING SOLUTIONS BY EXTRACTION WITH ACETONE Robert R. Dougherty, Minneapolis, Minn., assignor, by

mesne assignments, to Control Data Corporation, Minneapolis, Minn., a corporation of Delaware Filed Feb. 23, 1968, Ser. No. 707,837 Int. Cl. B01d 11/04; C01g 37/12, 37/14 US. Cl. 23-312 7 Claims ABSTRACT OF THE DISCLOSURE Spent or used aqueous chromic acid etchants are treated to separate chromic acid from by-products of the etching process which are dissolved or suspended in the spent etchants, thereby permitting the chromic acid to be reused and permitting the by-products of the etching process to be concentrated or isolated for waste disposal and/or recovery of the metal values contained therein. In a typical process, chromic acid and Water are separated from the by-products by acetone extraction. The extract is then stripped of the acetone to produce an aqueous solution of chromic acid which can be reused in the etching process. The raffinate is rich in the by-products of the etching process.

BACKGROUND OF THE INVENTION Chromic acid solutions are in widespread use, particularly as etchants for copper in the manufacture of printed circuits. These chromic acid etching solutions can be purchased ready for use (with or without dilution), or can be prepared by the user from proprietary ingredients supplied by various manufacturers. In either event, the resulting etching solutions (i.e. etchants) will typically contain water, chromic acid, and sulfuric acid, as well as various proprietary ingredients (e.g. foam suppressants) and by-products from the formation of the fresh chromic acid (e.g. sodium sulfate).

In use, the etchant is applied to (e.g. by spraying) or otherwise contacted with a previously prepared (e.g. partially masked) copper-clad board or film which is to be etched. Frequently, the etchant will be preheated (e.g. 160 F.) and brought into contact with the supported copper sheet to be etched by spraying the hot etchant onto the copper sheet. However, the etching can be performed by simply dipping the supported copper sheet into a bath of the etchant. In either event, the process will be continued until the desired amount of copper has been removed by the etching process. Then, the etched copperclad board or film is usually Washed, with or without a neutralizing agent, to thereby remove any remaining acids from the surfaces of the board or film and thereby terminating the etching process. Finally, the masked or protected areas of the copper-clad board or film are treated to remove the masking or protective agent which has been used to prevent selected portions of the copper coating from being etched.

As a result of the etching process, the etching solution which is used becomes contaminated with by-products of the etching process. With chromic acid etchants containing both chromic acid and sulfuric acid, these byproducts Will typically be various metal salts and oxides such as, for example, cupric sulfate (CuSO and chromic oxide (Cr O Precise identification of the by-products is difficult, particularly where proprietary etchants of unknown composition have been used.

When the etchant becomes too weak and too contaminated to perform satisfactorily, it will be replaced with a fresh chromic acid etching solution. At the present time, it is common for the spent or used etching solutions to be disposed of by discharging them into streams, septic tanks, public sewers, or occasionally into dry wells. Spent chromic acid etchants are not generally considered to have any particular salvage value even though they contain chemicals of value (e.g. chromic acid and copper metal present in chemically combined forms such as copper sulfate).

SUMMARY OF THE INVENTION It has now been discovered, and this discovery forms one basis of the present invention, that spent aqueous chromic acid etchants can be treated or processed to separate or remove by-products of the etching process which are dissolved in or suspended in the spent etchants. At the same time, a cleaner aqueous solution of chromic acid is obtained which can be recovered and reused in the etching process.

The desired treatment of spent aqueous chromic acid etchants is accomplished by mixing the spent etchant with a water-miscible or water soluble ketone (e.g. acetone) which functions as an extractive solvent. Chromic acid and considerable water are removed in the extract phase, while the rafiinate contains the by-products of the etching process and the remainder of the water.

The ketone (e.g. acetone) can be separated from the chromic acid, in the extract phase, thereby producing a cleaner, re-usable aqueous solution of chromic acid. This solution can be used as is for etching purposes, can be activated by the addition of sulfuric acid, or can be mixed with fresh chromic acid etchant.

As a result of the present process, the by-products which are separated from the spent or used chromic acid etchants are more concentrated than before, thereby simplifying disposal. Still further, these concentrated byproducts (i.e. the residue) contain (or are mixed with) substantially less chromic acid than before. As a con- .sequence, it is possible to further separate the by-products of the etching process by techniques which are not operable in the presence of substantial amounts of chromic acid. For example, the residue can be mixed with methanol and filtered to remove solid copper-containing particles while keeping chrominum compounds (i.e. by-products) in solution. In this manner, the chemical by-products can be recovered in forms having re-sale value and need not be disposed of, as by being discharged to the sewer. This latter feature is an important anti-pollution control means since copper salts (e.g. copper sulfate) often have an undesirable effect upon fish and other living organisms present in rivers and streams.

THE DRAWINGS FIG. 1 is an isometric view of apparatus which can be used to etch copper plates, as in the manufacture of printed circuits.

'FIG. 2 is a flow diagram illustrating the treating process of the present invention.

DETAILED DESCRIPTION The etching of copper metal, including its alloys, is a part of the manufacturing procedures used in many industries. One such application is in the manufacture of printed circuits.

In the manufacture of printed circuits, a thin copper sheet or foil is bonded to or laminated with rigid or flexible non-conductive supporting substrate (e.g. a hard or rigid plastic or resinous board or sheet of phenolformaldehyde resin). Alternatively, other copper coating techniques (e.g. vacuum metallization) can be used to achieve similar results. The desired circuit is then printed on the copper sheet, as by the use of photographic or silk screen processes. The ink-like compositions used in this printing process are coating compositions which, upon drying or curing, are resistant to attack by com- 3 mon etching solutions (e.g. resistant to aqueous chromic acid etchants). For this reason, such compositions are commonly referred to as resists.

After the desired circuit has been printed on the copper foil or sheet with a resist, the supported copper sheet is ready for the etching process.

Referring now to FIG. 1, one type of apparatus has been shown which can be used to perform the etching process. The apparatus consists of a tank or reservoir 1, a supporting shield 2 extending upwardly from one side of tank 1, a supporting rack 3 caried by shield 2 and used to support and position one or more copper sheets 4 (previously described) which are to be etched. Tank 1 is also provided with an outlet hose 5 and pump 6 for withdrawing etchant (e.g. aqueous chromic acid) from tank 1 and delivering it through hose 8 to spray gun 9 where it can be sprayed onto the copper panels or sheets 4 which are to be etched. If desired, heating means (not shown) can be provided to heat etchant 7 to any desired temperature.

In operation, copper sheets or panels 4 are prepared in the manner previously described. These panels are then hung or otherwise fastened upon supporting arm 3. The etching solution, frequently heated, is then sprayed onto panels 4 using spray gun or nozzle 9. This process is continued until the desired amount of copper has been etched or eaten away from panels 4. The panels 4 are then removed from supporting rack 3 and washed to remove and/ or neutralize any etchant which may remain upon the panels 4. Then, the resist is removed (e.g. by rubbing with a rag soaked in a suitable organic solvent). The etched copper panels are then finished or processed by any of a variety of known methods to produce printed circuits of the type known to the art.

When aqueous chromic acid is used as the etchant, fresh etchant will often contain both chromic acid and sulfuric acid. However, as a result of the etching process, various by-products are formed such as cupric sulfate and chromic oxide (Cr O Other by-products are undoubtedly formed as well. The exact nature of these by-products will depend upon the particular ingredients used to formulate the original or fresh etchant and the conditions of use of the etchant.

As a result of the etching process, the concentration of chromic acid in bath 7 will decrease and the concentrations of the various by-products will increase, all with respect to time of use. Frequently, these changes can be observed as a darkening of bath 7 during use. Also, the rate of etching (i.e. the rate of copper removal) will decrease or diminish with use. As a consequence, the chromic acid etchant must be replaced at some point in time if a satisfactory rate of etching is to be maintained. The extent to which the activity of the etchant can be permitted to decrease without replacement will depend upon various factors, including economics, type of equipment, and, to a certain extent, individual preference. With some of the more commonly used chromic acid etchants, it is common for the etchants to be used or re-used until the etchant contains at least 4 ounces (oz.) of copper metal per gallon. Although some users will continue to re-use the etchant until the level of copper metal is as high as 12 or 14 oz. of copper metal per gallon of etchant, it is more common for the etchants to be replaced when they contain 5-10 oz. of copper metal per gallon of etchant (e.g. 6-8 oz. of copper metal on the same basis).

The copper content of etchants can be determined by reducing the copper from the cupric ion to the cuprous ion (e.g. by the addition of sodium sulfite) and then forming a precipitate of cuprous t-hiocyanate (e.g. by the addition of potassium thiocyanate). The weight of the precipate, after Washing and drying, is then used to calculate the copper content of the etchant.

As previously indicated, it is a present common practice to merely dispose of the spent chromic acid etchants as valueless Waste materials.

However, the present invention provides a means The recovery process of this invention can be understood by referring to FIG. 2 in conjunction with the following description.

Spent aqueous chromic acid etchant containing metal containing by-products of the etching process is withdrawn from reservoir 10 and mixed with a water-mis cible or water-soluble ketone (e.g. acetone) from reservoir 11. A mixing head 12 has been shown for this purpose. This mixture is then fed to or introduced to 21 separation vessel 13 which include sa separation zone. Vessel 13 can take various forms, although an open tank will suffice. Desirably, separation vessel 13 will be equipped with some means (not shown) for continuously or periodically vibrating or agitating the mixture which has been introduced into vessel 13 from mixing head 12. An ultra-sonic vibrator can be used for this purpose. The use of vibrating or shaking means is preferred over vigorous agitation (i.e. the type which completely disrupts the separation of the phases). However, vigorous agitation can be used if its use is followed by a quiescent period or a period of mild motion. The mixture introduced into separation vessel 13 will separate with time (e.g. five minutes) into an upper or extract phase 14 and a lower or raftinate phase 15. It has been observed that the extract phase 14 will contain a significant quantity of chromic acid and substantially reduced or even insignificant quantities of the metal-containing by-products of the etching process (e.g. a reduced copper content).

The raffinate phase 15 is withdrawn from the separation vessel 13 (e.g. at the bottom) through line 16 and can be disposed of as sewage, or more preferably, subjected to a further treatment to separate the copper and chromium metal values contained therein. Such a separation process can be conducted as an integral part of the present process or performed separately, as by a different business concern engaged in the salvage of waste products. The raflinate phase 15 will typically appear as a watery dark green or black sludge or precipitate and may contain some visually observable suspended or floating small solid particles or cyrstals.

The upper or extract phase (frequently amber or red in color) consisting essentially of acetone, chromic acid, water and probably some sulfuric acid is then removed from separation vessel 13 and delivered to separation vessel 17. Typically, separation vessel 17 will be distillation apparatus. In vessel 17, the acetone is removed from extract phase 14 and can be recycled through line 18 to vessel 11. The aqueous chromic acid which remains is withdrawn from vessel 17 through line 19. This reclaimed or recovered chromic acid can then be reused in the etching process shown in FIG. 1. Frequently, the chromic acid which has been recovered will be activated by the addition of some sulfuric acid.

The treating agents or extractive solvents which can be used in the process of this invention are the watermiscible or water-soluble ketones. By water-miscible or water-soluble, it is meant to include only those ketones which have a solubility in water at 25 C. of at least 20 grams of ketone per milliliters of water. More desirably, the ketone which is selected will be even more soluble in water than that (e.g. infinitely soluble in'water). Suitable ketones are known to include acetone and methyl-ethyl ketone (i.e. Z-butanone). Inoperable acetones include methyl-isopropyl ketone, 2-hexanone, S-methyl- 2-hexanone, and benzyl-l-naphthyl ketone. Acetone is far superior to methyl-ethyl ketone and is the preferred ketone because of its effectiveness and low cost. Mixtures of ketones can be used.

The amount of ketone which will be used will be an effective amount, usually above 0.5 part by volume of ketone per part by volume of spent chromic acid etchant. The speed and effectiveness of the separation are greater when more than 0.9 volume of ketone are used per volume of spent chromic acid etchant. However, above volumes of ketone per volume of spent etchant, the process becomes unwieldy. Optimum results have been obtained using a ketone: spent etchant volume ratio of 1-5 :1, preferably 2-4: 1.

The separation which takes place within the separation zone of vessel 13 can be conducted under ambient conditions of temperature and pressure. However, higher and lower temperatures can be used, as can sub-atmos pheric and super-atmospheric pressure. For convenience, it is preferred to operate at approximately atmospheric pressure (or autogenous pressure it a closed separation vessel is used) and at temperatures of 590 C. (e.g. 15 -75 C.). However, higher and lower temperatures (e.g. minus 15 C.) can be used.

The time required to make the separation in vessel 13 can vary substantially, depending upon the size and configuration of the separation vessel employed, the relative amounts of ketone and spent etchant, and the degree of agitation. Using a 3:1 volume ratio of acetone and spent chromic acid etchant, together with ultra-sonic vibration at room temperature (about 25 C.) separation can be extremely rapid (e.g. 2-5 minutes). However, the degree of separation and by-product concentration increases as more and more time is permitted for the separation to take place. The increase in degree or extent of separation can be observed as an increase in the size of the upper or extract phase 14 (see FIG. 2) and a consolidation or decrease in size of the rafiinate phase 15 (also shown in FIG. 2).

The present invention will be further understood by reference to the following specific examples which include a preferred embodiment. Unless otherwise indicated, all parts and percentages are by weight.

Example I Supported copper sheets were etched as part of the process of preparing printed circuits using a commercially available chromic acid etchant (Etchant CRM sold by Shipley Company, Inc. of Newton, Mass.) The etchant was reused until it contained about 6-7 ounces of copper metal per gallon of etchant and had an unsatisfactory etching rate. It was then considered spent and was removed for processing according to this invention.

Thirty millilters of this spent aqueous chromic acid etchant, believed to contain chromic acid, sulfuric acid, cupric sulfate and chromic oxide, were mixed with 90 milliliters of technical grade acetone (i.e. a 3:1 volume ratio of ketone to spent etchant). This mixture was permitted to stand at room temperature for several minutes under the influence of ultra-sonic vibrations transmitted through a water bath. Almost immediately, a visibly observable upper extract phase was formed. After 2-5 minutes, most of this upper or extract phase, amberred in color, was separated from the lower raffinate phase (a watery dark green or black gummy sludge) by decanting. The removed extract phase, amounting to approximately 97 milliliters, was then distilled at substantially atmospheric pressure and a 6070 C. vapor temperature at the inlet to an overhead condenser (cooled with cold water). After 77 milliliters of acetone had been collected from a trap on the condenser, the vapor temperature began to rise, acetone ceased to be evolved, and so distillation has halted. The material which remained in the distillation flask amounted to approximately 20 milliliters. This material contained chromic acid, some sulfuric acid, a substantial amount of water, and a small amount of acetonel This residual material (i.e. aqueous chromic acid separated from by-products of etching) was activated by the addition of 3 milliliters of sulfuric acid (94% or 66 Baum) and then used as an etchant for copper in the manufacture of printed circuits. The reclaimed etchant performed satisfactorily.

The lower or rafiinate phase from the acetone extraction =(i.e. the watery residue, precipitate or sludge) was further processed by the addition to the residue of 50 milliliters of methanol. This mixture was then filtered to remove solid particles which were rich in copper (probably as cupric sulfate). These solid particles were then washed twice with 10 milliliter increments of methanol. The washed solids were then oven-dried at 210 F. for approximately 2 hours. After drying, 7 grams of solid material were obtained. This material contained substantial amounts of copper, together with various impurities (eg sodium sulfate). The presence of substantial amounts of copper in this solid product was verified by chemical analysis and by redissolving the solid material in water and electroplating copper from the solution The remaining methanol-rich liquid was then mixed with the two 10 milliliter methanol washes and the resulting mixture placed in a distillation unit operating at substantially atmospheric pressure and a vapor temperature of approximately 6-5 -70 C. Methanol was recovered from the overhead condenser. A small amount of liquid remained in the distillation unit which was rich in chromium, probably as chromic oxide (Cr O Example II When the chromic acid recovery procedure of Example I was repeated using methyl-ethyl ketone as the treating agent or extractive solvent instead of acetone, aqueous chromic acid was also recovered. The reclaimed aqueous chromic acid was suitable for reuse as an etchant for copper. However, the rate of separation and extent of separation of chromic acid from the by-products of the etching process was not nearly as great as was obtained when acetone was used as the extractive solvent.

Example III The general procedure of Example I was repeated using acetone. During processing, various samples were taken and submitted to chemical analysis with the following results Reclaimed etchant: Percent Chromium (+6) 6.35 Chromium (+3) 3.10 Copper 0.09 Sulfur 1.10

Copper-containing solids from methanol wash:

Copper 33.8 Sulfur 17.6

Sodium r (1) A minor constituent (i.e. 110%).

Although the present invention has been described with a certain degree of particularity, it will be realized that various changes and modifications can be made to the invented process without departing from the spirit and scope of the invention.

What is claimed is:

1. The process of treating used aqueous chromic acid copper etching solutions which comprises:

(a) mixing 2-4 volumes of acetone per volume of used aqueous chromic acid etching solution with a used aqueous chromic acid etching solution which contains by-products of the etching process including copper ions;

(b) agitating the resulting mixture to form an extract phase and a raffinate phase, said rafiinate phase containing by-products of the etching process;

(c) separating the extract phase containing extracted chromic acid from the raffinate phase; and

(d) removing acetone from the separated extract phase to recover extracted chromic acid.

2. The process of claim 1 including the additional step of recovering the copper from the raffinate phase.

3. The process of claim 1 wherein the extract phase and rafiinate phase are formed at a temperature of 15- 75 C.

4. The process of claim 1 wherein the formation of the extract and rafiinate phases is assisted by the use of vibration.

5. The process of claim: 1 wherein the recovered chromic acid is susbsequently recycled to etch copper.

6. The process of claim 4 wherein sulfuric acid is admixed With the recovered chromic acid, and the resulting mixture is then recycled to etch copper.

7. The process of claim 6 wherein the used etching solutions have been used to etch copper films and sheets in the manufacture of printed circuits and wherein the recovered chromic acid is recycled to etch copper films and sheets in the manufacture of printed circuits.

8 References Cited UNITED STATES PATENTS 2,600,171 6/1952 Sagen 23-145 X 3,458,278 7/1969 Bonnivard 23-145 X FOREIGN PATENTS 550,395 9/ 1956 Belgium 698,700 11/1964 Canada.

OTHER REFERENCES Kemp, Journal of the American Chemical Society, July 5, 1967; 89:14, pp. 3433 to 3438.

Ledford, Industrial and Engineering Chemistry, January 1955, pp. 83 to 86.

Weinhardt, Industrial and Engineering, July 3, 1951, 43:7, pp. 1676-1684.

West Metallurgia, July 1956, pp. 47 to 48.

NORMAN YUDKOFF, Primary Examiner S. J. EMERY, Assistant Examiner US. Cl. X.R. 

